After 10 orbits of the Moon, the engine of the Service Propulsion System (SPS) re-fired to put Apollo 8 on course for home. If the engine had failed, the spacecraft would have been stranded in lunar orbit, and the crew would have perished. Fortunately, nothing of the sort happened. The first men to orbit the Moon returned home safely to a rousing welcome. The Apollo spacecraft consisted of three parts or modules. The crew of three was housed in the Command Module (CM). This was cylindrical in shape. At its nose was a docking tunnel through which astronauts could crawl into other spacecraft whenever required to.
Attached to the base of the Command Module was the Service Module (SM). This was a cylinder, 7.5 m long, containing air, water, fuel and power supplies, as well as the Service Propulsion System. The third part of the spacecraft was the Lunar Module (LM), a spidery, four-legged craft that was to descend to the lunar surface with two astronauts. Before a landing on the Moon could be attempted this crucial part of the spacecraft had to be rigorously tested to make sure that it could separate and return and dock with the mother ship without a hitch. This comprised the main mission of Apollo 9 and then, Apollo 10.